Most light automotive vehicles, such as automobiles, sport-utility vehicles, vans and light trucks, have four wheels, but in the typical vehicle the driving engine which propels the vehicle is coupled to only two of the wheels. In older vehicles the rear wheels normally propelled the vehicle, but in newer vehicles it is commonly the front wheels. Light trucks and off-road vehicles commonly have four-wheel drive (4WD), the famous U.S. Army Jeep for example, but often operate with power delivered to only two wheels. If the need arises for more traction, the transmission output is coupled with the other two wheels through a manually operated transfer case. The rear and front wheels share the torque delivered by the driving engine under a fixed gear ratio.
In recent years automotive manufacturers have produced more sophisticated vehicles with all wheel drive (AWD). In the typical vehicle of this type, all four wheels normally drive the vehicle with the engine torque split between the front and rear wheels. The driving engine delivers power through a transmission which is in turn connected directly to two of the wheels, referred to as the primary driving wheels. The remaining two wheels, referred to as the secondary driving wheels, are connected to the transmission through a torque coupling which accommodates slight variations in speed between the primary and secondary wheels.
While a differential is commonly interposed between the primary driving wheels and the transmission, the connection is considered “direct” in the sense that no slippage can develop between the primary wheels and the transmission. While a second differential is commonly interposed between the torque coupling and the secondary wheels, the connection is “indirect” as the torque coupling allows for slippage between the secondary wheels and the transmission. The torque coupling operates to divides the torque between the primary and secondary wheels.
Torque couplings are used in four-wheel-drive or all-wheel-drive applications to transfer torque to the secondary axle or to provide a limited slip differential function between a pair of wheels on a single axle under operating conditions where one wheel on the axle looses traction. An essential characteristic of a torque coupling is the capability of modulating the torque delivered by the driving engine, in order to improve vehicle handling, safety, and acceleration performance, even under no-wheel-slip conditions and at higher vehicle speeds. There are basically two functions to be performed by a torque coupling in a four-wheel or all-wheel drive application. First, traction enhancements at low speed and high torque operating conditions, and second, vehicle dynamic control at high speed and low torque operating conditions.
One type of torque coupling is a hydraulic coupling. Hydraulic couplings consist essentially of a wet-plate clutch and a pumping mechanism supplying pressure required for the clutch actuation.
Usually, as is shown in U.S. Pat. No. 5,595,214 to Shaffer et al., and in U.S. Pat. No. 5,469,950 to Lundström et al., the pumping mechanism is of the gerotor or geared pump type, or the piston/axial cam type, and is configured to take advantage of the differential speed resulting from slip between either a pair of axles of the vehicle, or between each wheel on a common axle, to provide the pressure to actuate the clutch.
The overall coupling responsiveness depends upon the speed with which the hydraulic pressure is created and how rapidly the pressure can be applied to the clutch. As is well known, the hydraulic fluid volume flow achieved by the pumping mechanism and the directly proportional hydraulic fluid pressure differential each depend on the relative speed between the pumping mechanism components.
Accordingly, it would be desirable to provide a coupling device, such as a hydraulic coupling, with improved responsiveness for torque modulation, and which is capable of maximizing torque transfer from the drive motor to the driven components when torque modulation is not required.